Fuel cell-powered vehicles depend on electric power from hydrogen and oxygen as fuels and have attracted attention as the next-generation clean vehicles, which do not emit such hazardous substances as carbon dioxide [CO2], nitrogen oxide [NOx] and sulfur oxide [SOx], unlike the current conventional gasoline engine vehicles or diesel engine vehicles. In Japan, the introduction of 5 million such vehicles prior to 2020 is planned under the leadership of the Japanese Ministry of Economy, Trade and Industry.
At present, the greatest problems to be solved before the practical use of these fuel cell-powered vehicles are how to generate the fuel, i.e., hydrogen, and how to store it. Various research and development work is going on at the present time.
Typical methods are loading a hydrogen gas cylinder into the vehicle, generating hydrogen by reforming methanol or gasoline in a reformer carried on the vehicle, and installing a hydrogen storage alloy with hydrogen adsorbed therein in the vehicle.
While each of these methods has its merits and demerits, fuel cell-powered vehicles carrying a hydrogen gas cylinder, were first put on the world market by Japan in December 2002 (Heisei 14), and several of them are already in use as official cars by the Japanese Ministry of Land, Infrastructure and Transport and so on.
However, while the current fuel cell-powdered vehicles are already performing close to the standard of gasoline-driven private cars with a maximum speed of about 150 km/hr and power of about 100 horsepower, the maximum range is less than 300 km due to the limited cylinder size, and this problem has prevented them from coming into wide use.
The method for installing a reformer, which uses methanol or gasoline as a fuel, still has some problems; for example, methanol is toxic and the gasoline needs to be desulphurized. Also an expensive catalyst is required at the present time and, further, the reforming efficiency is unsatisfactory, hence the CO2 emission reducing effect does not justify the increase in cost.
The method which uses a hydrogen storage alloy has technological problems. For example the hydrogen storage alloy is very expensive, and excessive time is required for hydrogen absorption, which corresponds to fuel charging, and the hydrogen storage alloy deteriorates by repeating absorption and releasing hydrogen. Therefore the great deal of time is still required before this method can be put into practical use.
With the background discussed above, various research and development work is being encouraged in Japan in order to improve the performance of the fuel cell-powered vehicles carrying a high-pressure gas cylinder, and also reduce the cost of its production. In order to popularize the so-called next-generation clean vehicles, it is necessary to overcome the following problems.
The range of the fuel cell-powered vehicles should be increased. The infrastructure for example, the hydrogen stations necessary for the popularization of the car should be prepared. And the technology to improve the safety in handling of hydrogen should be developed.
A trial calculation indicates that, in order to extend the range of the vehicle to 500 km, for instance, the hydrogen gas pressure in the cylinder to be carried on the vehicle should be increased from the current level of 35 MPa to a higher level of 70 MPa. Further, hydrogen gas stations become necessary instead of the existing gasoline stations and, accordingly, the generation, transportation and storage of high-pressure hydrogen gas, as well as rapid charging (feeding to vehicles) thereof, become necessary.
Since hydrogen gas is flammable, close attention should be paid in handling it. As for the interaction between hydrogen gas under very high pressure exceeding 50 MPa in particular, and the structural equipment members, there are a number of points that remain unclear, hence it is imperative that the technology for the safe utilization of equipment be established.
The material used in the high-pressure hydrogen gas equipment in the fuel cell-powered vehicles commercialized in 2002 (Heisei 14) is an austenitic stainless steel, i.e., JIS SUS 316 type material, whose reliability has been widely recognized in the art. This is because this steel has better hydrogen embrittlement insusceptibility, in an environment of up to 35 MPa hydrogen than other structural steels such as JIS STS 480 type carbon steel and SUS 304 type stainless steel, and also is excellent in workability and weldability, and the technology of its utilization has been established.
However, in using this SUS 316 steel as piping for high-pressure hydrogen gas, whose gas pressure has been increased from 35 MPa to 70 MPa, the outer diameter of the pipe should be increased to 34.7 mm, the inner diameter to 20 mm (pipe wall thickness 7.35 mm), for instance, as compared with the conventional outer diameter of 26.2 mm and the inner diameter of 20 mm (wall thickness 3.1 mm). Thus, the piping cannot endure unless the pipe wall thickness is increased twice or more and the weight three times. Therefore, a marked increase in on-board equipment weight and in size of gas stations will be inevitable, presenting serious obstacles to practical use.
It is known that cold working increases the strength of austenitic stainless steel. Therefore it is possible to avoid the increase in the pipe wall thickness by increasing the strength with such cold working as drawing and rolling.
High-level strength can be obtained by such cold working. However the ductility and toughness markedly decrease and, further, an anisotropy problem may arise due to such working. In addition, it has been made clear that cold-worked austenitic stainless steel shows a marked increase in hydrogen embrittlement susceptibility in a high-pressure hydrogen gas environment, and it has been found that, considering the safety in handling high-pressure hydrogen gas, cold working cannot be employed for increasing pipe strength.
As for the method of strengthening austenitic stainless steel, the so-called solid solution hardening method, in which a large amount of nitrogen [N], as a solid solution element is used, is known from Japanese Patent Laid-open (JP Kokai) Nos. H05-65601 and H07-188863. Further, in JP Kokai No. H05-98391, there is proposed a precipitation hardening method, which comprises causing precipitation of carbides and/or nitrides. However, these conventional strengthening technologies inevitably decrease ductility and toughness and, in particular, cause an increase in anisotropy in toughness, possibly leading to the same problem as in the cold working when the pipes are used in a high-pressure hydrogen gas environment.
Furthermore, in JP Kokai No. H06-128699 and JP Kokai No. H07-26350, there are proposed stainless steels, in which corrosion resistance is improved by adding a large amount of nitrogen [N]. However, these steels do not have characteristics to cope with a high-pressure hydrogen gas environment; hence it is not easy to secure the safety for the same reasons as mentioned above.
Hydrogen gas stations may be located in seashore regions. Vehicles may also be exposed to a salt-containing environment while running or parking. Therefore, the material to be used for hydrogen gas storage containers is also required to be free of any fear of stress corrosion cracking due to the chloride ion.
One of the means for improving the stress corrosion cracking resistance of stainless steel is increasing Cr content. However, merely increasing the Cr content causes precipitation of large amounts of Cr nitrides and the sigma phase. Therefore, such steel cannot have the characteristics required for steel materials for high-pressure hydrogen gas.
The containers and piping for high-pressure hydrogen and accessory parts or devices that belong thereto are often manufactured by welding. The welded joints also have the following problems. Namely, a decrease in strength occur in the weld metal of the joints due to melting and solidification, and in the welding heat affected zone due to heat cycles in welding. This decrease in the strength in the welding heat affected zone can be prevented by carrying out appropriate heat treatment after welding. However, the weld metal has a coarse solidification structure, and, therefore, the strength thereof cannot be improved by mere post-welding heat treatment.